U.S. patent application number 11/532296 was filed with the patent office on 2007-03-22 for light source unit for use in a lighting apparatus.
Invention is credited to Masatake Baba.
Application Number | 20070064421 11/532296 |
Document ID | / |
Family ID | 37898439 |
Filed Date | 2007-03-22 |
United States Patent
Application |
20070064421 |
Kind Code |
A1 |
Baba; Masatake |
March 22, 2007 |
LIGHT SOURCE UNIT FOR USE IN A LIGHTING APPARATUS
Abstract
A backlight source of a LCD device includes constant-power red,
green and blue LEDs emitting light having a constant output power,
and a tone-correction red LED emitting light having a controlled
output power controlled based on the temperature of the backlight
source. The tone-correction red LED has a larger output power upon
a temperature rise to supplement the reduced output power of the
constant-power red LED.
Inventors: |
Baba; Masatake;
(Nakahara-ku, JP) |
Correspondence
Address: |
Norman P. Soloway;HAYES SOLOWAY P.C.
Suite 140
3450 E. Sunrise Drive
Tucson
AZ
85718
US
|
Family ID: |
37898439 |
Appl. No.: |
11/532296 |
Filed: |
September 15, 2006 |
Current U.S.
Class: |
362/231 |
Current CPC
Class: |
G02B 6/0068 20130101;
H05B 45/22 20200101; H05B 45/28 20200101 |
Class at
Publication: |
362/231 |
International
Class: |
F21V 9/00 20060101
F21V009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 21, 2005 |
JP |
2005-273994 |
Claims
1. A light source unit comprising: a plurality of
light-emitting-diodes (LEDs), said LEDs including at least one
constant-power LED and at least one tone-correction LED; and a
power control unit for controlling an output power of said
tone-correction LED based on a temperature of said lighting
apparatus, and maintaining the output power of said constant-power
LED with respect to the temperature of said lighting apparatus.
2. The light source unit according to claim 1, wherein said at
least one constant-power LED include red, green and blue LEDs.
3. The light source unit according to claim 2, wherein said at
least one tone-correction LED includes a tone-correction red LED,
and said power control unit increases the output power of said
tone-correction red LED upon a temperature rise.
4. The light source unit according to claim 2, wherein said at
least one tone-correction LED include tone-correction red and green
LEDs, and said power control unit increases the output power of
said tone-correction red and green LEDs upon a temperature
rise.
5. The light source unit according to claim 2, wherein said at
least one tone-correction LED includes a tone-correction blue LED,
and said power control unit reduces the output power of said
tone-correction blue LED upon a temperature rise.
6. The light source unit according to claim 1, further comprising
an input device for receiving a control signal controlling the
output power of said constant-power LED and/or tone-correction
LED.
7. A lighting apparatus comprising a light source unit and an
optical guide plate for receiving light emitted by said light
source unit and emitting said received light, said light source
unit including: a plurality of light-emitting-diodes (LEDs), said
LEDs including at least one constant-power LED and at least one
tone-correction LED; and a power control unit for controlling an
output power of said tone-correction LED based on a temperature of
said light source unit, allowing the output power of said
constant-power LED to be fixed with respect to the temperature of
said light source unit.
8. A liquid crystal display device comprising: a lighting apparatus
for emitting light; and a liquid crystal display panel for
receiving light emitted by said lighting apparatus as backlight,
said lighting apparatus comprising a light source unit and an
optical guide plate for receiving light emitted by said light
source unit and emitting said received light, said light source
unit including: a plurality of light-emitting-diodes (LEDs), said
LEDs including at least one constant-power LED and at least one
tone-correction LED; and a power control unit for controlling an
output power of said tone-correction LED based on a temperature of
said lighting apparatus, and maintaining the output power of said
constant-power LED with respect to the temperature of said lighting
apparatus.
Description
BACKGROUND OF THE INVENTION
[0001] (a) Field of the Invention
[0002] The present invention relates to a light source unit for use
in a lighting apparatus and, more particularly, to a light source
unit including light emitting diodes (LED).
[0003] (b) Description of the Related Art
[0004] In general, a LED or cold-cathode tube is used as a light
source in a lighting apparatus such as a backlight unit in a liquid
crystal display (LCD) device. The cold-cathode tube, which
generally includes a dedicated ignition circuit and thus
necessitates use of a higher voltage, has been used mainly in a
large-dimension LCD device such as in a television or monitor set.
On the other hand, the LED, which has a relatively lower brightness
and a smaller occupied area, has been used mainly in a
small-dimension LCD device such as in a cellular phone or personal
data assistant (PDA). However, development of LEDs to have a larger
output power and a higher luminance efficiency has allowed the LEDs
to be used as backlight units in large-dimension LCD devices.
[0005] It is noted that the LED continuously emitting light in a
long time interval is subjected to a higher device temperature to
thereby lower the output power thereof. The degree of reduction in
the output power associated with the higher device temperature
generally depends on the materials used in the p-n junction of the
LED, and is thus different between red, green and blue LEDs in the
backlight unit. This may cause a chromaticity shift in a color LCD
device using the LEDs in a backlight unit
[0006] In a literature entitled "RGB-LED Backlighting Monitor/TV
for Reproduction of Images in Standard and Extended Color Spaces",
IDW '04 p. 683-686, the lighting apparatus described therein
enables correction of chromaticity and brightness. FIG. 5 shows the
configuration of the lighting apparatus described in the
literature. The lighting apparatus 40 includes an optical guide
plate 41, and a light source unit 42 disposed along a longer edge
surface of the optical guide plate 41.
[0007] The light source unit 42 includes a substrate 45 having a
main surface opposing the longer edge surface 41a of the optical
guide plate 41, and LEDs 46 formed on the main surface of the
substrate 45 and each emitting a red, green or blue light
therefrom. The light emitted by the LEDs 46 is introduced through
the longer edge surface 41a into the internal of the optical guide
plate 41, and scattered uniformly within the optical guide plate
41, to be emitted through the front surface of the optical guide
plate 41 in the direction normal to the sheet of the drawing.
[0008] The lighting apparatus 40 further includes an optical sensor
unit 43 disposed on the other longer edge surface of the optical
guide plate 41 opposing the lighting apparatus 40, an LED driver 44
for driving the light source unit 42, and a temperature sensor unit
47 disposed on the substrate 45 for detecting the substrate
temperature. The optical sensor unit 43 has a function of detecting
the intensity of the red, green and blue lights separately.
[0009] The LED driver 44 monitors the intensity of each of red,
green and blue lights detected by the optical sensor unit 43, and
controls the output power of the LEDs 46 so that the lighting
apparatus 40 outputs specified chromaticity and brightness. The LED
driver 44 also monitors the substrate temperature of the lighting
apparatus 40 measured by the temperature sensor 47, to correct the
change of output power of the LEDs 46 caused by the temperature
change of the substrate 45.
[0010] The lighting apparatus 40 described in the literature uses
calculation of the output power of the LEDs 46 based on the signals
supplied from the optical sensor 43 and the temperature sensor 47.
Calculation for the correction complicates the control of the power
to be supplied from the power source to the LEDs 46 and thus
necessitates use of a dedicated IC chip for the LED driver 44. In
addition, the cost of the optical sensor unit 43 raises the cost of
the lighting apparatus 40. The optical sensor unit 43 also has a
large occupied area to increase the dimensions of the lighting
apparatus 40. Thus, it is desired to simplify the lighting
apparatus which emits light having desired chromaticity and
brightness.
SUMMARY OF THE INVENTION
[0011] In view of the above, it is an object of the present
invention to provide a lighting apparatus suited to backlight
source unit capable of having a simplified structure and emitting
light having desired color and brightness.
[0012] It is another object of the present invention to provide a
light source unit for use in the lighting apparatus, and a LCD
device including the lighting apparatus.
[0013] The present invention provides a light source unit
including: a plurality of light-emitting-diodes (LEDs), the LEDs
including at least one constant-power LED and at least one
tone-correction LED; and a power control unit for controlling an
output power of the tone-correction LED based on a temperature of
the lighting apparatus, and maintaining the output power of the
constant-power LED with respect to the temperature of the lighting
apparatus.
[0014] The present invention also provides a lighting apparatus
including a light source unit and an optical guide plate for
receiving light emitted by the light source unit and emitting the
received light, the light source unit including: a plurality of
light-emitting-diodes (LEDs), the LEDs including at least one
constant-power LED and at least one tone-correction LED; and a
power control unit for controlling an output power of the
tone-correction LED based on a temperature of the light source
unit, allowing the output power of the constant-power LED to be
fixed with respect to the temperature of the light source unit.
[0015] The present invention also provides a liquid crystal display
device including a lighting apparatus for emitting light: and a
liquid crystal display panel for receiving light emitted by the
lighting apparatus as backlight, the lighting apparatus comprising:
a light source unit and an optical guide plate for receiving light
emitted by the light source unit and emitting the received light,
the light source unit including: a plurality of
light-emitting-diodes (LEDs), the LEDs including at least one
constant-power LED and at least one tone-correction LED; and a
power control unit for controlling an output power of the
tone-correction LED based on a temperature of the lighting
apparatus, and maintaining the output power of the constant-power
LED with respect to the temperature of the lighting apparatus.
[0016] In accordance with the light source unit of the present
invention, and the light source unit used in the lighting apparatus
and LCD device of the present invention, the chromaticity shift
and/or brightness change of the constant-power LED caused by a
temperature change of the light source unit can be cancelled by the
control of the output power of the tone-correction LED based on the
temperature. This is achieved without using an optical sensor in
the lighting apparatus, to thereby reduce the dimensions of the
lighting apparatus.
[0017] The above and other objects, features and advantages of the
present invention will be more apparent from the following
description, referring to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a schematic front view of a lighting apparatus
according to a first embodiment of the present invention.
[0019] FIG. 2 is a block diagram of the light source unit shown in
FIG. 1.
[0020] FIG. 3 is a graph showing the relationship between the
temperature of the p-n junction and the normalized output power of
the LEDs.
[0021] FIG. 4 is a block diagram of another light source unit.
[0022] FIG. 5 is a block diagram of a conventional lighting
apparatus.
PREFERRED EMBODIMENT OF THE INVENTION
[0023] Now, the present invention is more specifically described
with reference to accompanying drawings, wherein similar
constituent elements are designated by similar reference
numerals.
[0024] Referring to FIG. 1, a lighting apparatus, generally
designated by numeral 10, includes an optical guide plate 11, and a
light source unit 12 disposed along a longer edge surface 11a of
the optical guide plate 11. The light source unit 12 is configured
as a backlight unit for use in a LCD device. In FIG. 1, a LCD panel
19 in the LCD device is shown by a dotted line. The LCD panel 19 is
disposed in front of the lighting apparatus 10 to receive the light
emitted by the lighting apparatus 10.
[0025] The light source unit 12 includes a LED array 13, and a LED
driver 14 connected to the LED array 13. The LED array 13 includes
a substrate 15, a plurality of LEDs 16 arranged in a row on the
surface of the substrate 15 opposing the longer edge surface 11a of
the optical guide plate 11, and a temperature sensor 17 disposed on
the substrate 15 for detecting the substrate temperature of the
light source unit 12. The LED driver 14 is connected to the LEDs 16
and temperature sensor 17.
[0026] FIG. 2 is a block diagram of the light source unit 12 shown
in FIG. 1. The LEDs 16 shown in FIG. 1 include a plurality of
constant-power LEDs 21 which are driven at a constant power, and a
plurality of tone-correction LEDs 22 which are driven at a
controlled power controlled based on the temperature of the
substrate 15. The constant-power LEDs 21 includes red LEDs 21R,
green LEDs 21G and blue LEDs 21B, all of which emit respective
colored lights. The tone-correction LEDs 22 include red LEDs which
emit a red light. These LEDs 21, 22 are arranged at a constant
pitch on the substrate 15, and the tone-correction LEDs 22 are
disposed adjacent to the constant-power red LEDs 21R. It is to be
noted that the tone-correction LEDs 22 may be disposed on another
substrate.
[0027] In the present embodiment, the temperature sensor 17 is
configured by a thermistor, which assumes a resistance depending on
the temperature of the substrate 15. Use of the thermistor is
especially preferable because of the small dimensions and easy
arrangement thereof, thereby scarcely affecting the dimensions and
shape of the light source unit 12.
[0028] The LED driver 14 includes a power source circuit 31, a
correction circuit 32, and a current control circuit 33. The power
source circuit 31 connects to the constant-power LEDs 21R, 21G, 21B
and supply a constant power to each of the constant-power LEDs 21R,
21G, 21B.
[0029] The correction circuit 32 connects to the temperature sensor
17 and current control circuit 33, and includes therein a look-up
table which shows the relationship between the resistance of the
temperature sensor 17 and the amount of control to be supplied to
the current control circuit 33. The resistance of the temperature
sensor 17 in the look-up table may be replaced by a terminal
voltage of the temperature sensor 17 converted from the resistance.
The correction circuit 32 generates a control signal based on the
resistance that the temperature sensor 17 delivers as temperature
information of the lighting apparatus, by referring to the look-up
table.
[0030] The current control circuit 33 connects to the correction
circuit 32 and tone-correction LEDs 22. The current control circuit
33 receives the control signal from the correction circuit 32,
controlling the power to be supplied to the tone-correction LEDs 22
based on the received control signal. The control of the power
supplied to the tone-correction LEDs 22 may use a constant-current
control technique, a duty ratio control technique based on a pulse
width modulation (PWM) signal, and so on.
[0031] Back to FIG. 1, the optical guide plate 11 receives through
the longer edge surface 11a the light emitted by the light source
unit 12, scatters the received light within the optical guide plate
11, and emits the scattered light through the front surface thereof
toward the LCD panel 19 in the direction normal to the sheet of the
drawing.
[0032] FIG. 3 shows an example of the general relationship between
the output power of color LEDs and the temperature of the p-n
junction of the color LEDs. The output power plotted on ordinate is
normalized by the output power at a specific temperature (around 25
degrees), and shown in terms of percents. As understood from FIG.
3, the degree of reduction in the output power is larger in red
LEDs, medium in green LEDs and smaller in blue LEDs. The look-up
table stored in the correction circuit 32 is created based on the
relationship such as shown in FIG. 3, and is designed to redeem or
supplement the reduction in the output power of the constant-power
red LEDs 21R upon a temperature rise.
[0033] In FIG. 3, the green LED and blue LED experience a less
degree of reduction in the output power upon a temperature rise
compared to the red LED. Thus, in the present embodiment, the
tone-correction red LEDs 22 increase the output power thereof along
with the temperature rise, thereby correcting the tone or
chromaticity shift as well as the brightness reduction of the light
caused by the temperature rise. This provides stable chromaticity
and brightness for the light emitted by the lighting apparatus
10.
[0034] As described above, the output power of the tone-correction
red LEDs 22 is controlled based on the temperature of the substrate
15 to correct the tone and brightness shift of the light emitted by
the constant-power LEDs 21R, 21G, 21B. The lighting apparatus 10 of
the present embodiment obviates provision of an optical sensor and
a dedicated IC chip for correcting the chromaticity shift of the
emitted light caused by the temperature change. Thus, the tone and
brightness correction is achieved by a simple structure at a lower
cost.
[0035] It is to be noted that the tone-correction LEDs 22 may
further include one or more green LEDs and one or more blue LEDs,
for achieving the tone and brightness correction with a higher
accuracy and within a broader temperature range. In such a case,
the lighting apparatus 10 may include another correction circuit
and another current control circuit for the green LEDs and blue
LEDs.
[0036] If the tone-correction LEDs include red, green and blue
LEDs, the red LEDs may be operated in a higher temperature range
whereas the green and/or blue LEDs may be operated in a lower
temperature range. The tone-correction LEDs may include only blue
LEDs, which are controlled to reduce the output power thereof upon
a temperature rise, or in a higher temperature range.
[0037] FIG. 4 shows a block diagram of another light source unit 18
used in a lighting apparatus according to a second embodiment of
the present invention. The light source unit 18 is similar to the
light source unit 17 of FIG. 2 except that the light source unit 18
includes an input device 34. The input device 34 receives an input
signal from a user to deliver the same to the current control
circuit 33.
[0038] The current control circuit 33 is controlled by the input
signal to control the output power of the tone-correction LEDs 22
in addition to the control by the temperature signal supplied from
the temperature sensor 17 shown in FIG. 1. The other configuration
of the lighting apparatus of the present embodiment is similar to
that of the lighting apparatus 10 of the first embodiment.
[0039] The user of the lighting apparatus enters the input signal
through the input device 34 to control the current control circuit
33 for emitting a light having desired tone and brightness. The
input device 34 may deliver the input signal to the power source
circuit 31, to control the power source circuit such as 31 in FIG.
2 for controlling the output power of the constant-power LEDs 21R,
21G, 21B. The term "constant-power" as used herein means the power
which is constant with respect to the temperature change of the
lighting apparatus.
[0040] Since the above embodiments are described only for examples,
the present invention is not limited to the above embodiments and
various modifications or alterations can be easily made therefrom
by those skilled in the art without departing from the scope of the
present invention.
* * * * *